In complex listening situations such as cocktail parties where there are a number of competing sources, normal-hearing listeners are known to rely on a variety of acoustical cues for extracting the individual component sources from a pair of ear-input signals, e.g. spatial or pitch cues [Bregman, A. S. (1990), “Auditory Scene Analysis—The Perceptual Organization of Sound,” Cambridge, Mass.: The MIT Press, pp. 559-572, 590-594]. These cues may be conveyed by the detailed cycle-by-cycle or temporal fine structure properties as well as the more slowly varying temporal envelope properties of a waveform. Recent audiological research has shown that the ability of subjects with sensorineural hearing losses to make use of temporal fine structure-based information can be severely degraded, but that their sensitivity to temporal envelope-based information remains intact [Lorenzi, C., Gilbert, G., Carn, H., Garnier, S., and Moore, B. C. J. (2006), “Speech perception problems of the hearing impaired reflect inability to use temporal fine structure,” Proc. Natl. Acad. Sci. USA, 103, 18866-18869; Lacher-Fougëre, S., and Demany, L. (2005), “Consequences of cochlear damage for the detection of interaural phase differences,” J. Acoust. Soc. Am., 118, 2519-2526].
There is a large body of research dealing with human sound localization, which is reviewed in [Blauert, J. (1983), “Spatial Hearing,” Cambridge, Mass.: The MIT Press]. This research has shown that normal-hearing listeners can utilize across-ear or interaural differences in temporal fine structure (so-called interaural phase differences; IPDs) when localizing frequencies lower than about 1.5 kHz. In addition, it has shown that they can utilize interaural differences in the temporal envelope (so-called interaural envelope delays; IEDs) of more complex, amplitude-modulated signals. Generally speaking, listeners are relatively insensitive to IEDs below 1.5 kHz, but at higher frequencies (e.g. between 2-4 kHz) sensitivity to them is much better [Blauert, pp. 153-154]. Furthermore, listeners are less sensitive to IEDs within high-frequency, complex stimuli than they are to changes in IPDs within low-frequency stimuli [Bernstein, L. R. (2001), “Auditory processing of interaural timing information: New insights,” J. Neurosc. Res., 66, 1035-1046]. For complex broadband stimuli, therefore, IPDs seem to provide more potent localization information than IEDs (or interaural level differences for that matter [Wightman, F. L., and Kistler, D. J. (1992), “The dominant role of low-frequency interaural time differences in sound localization,” J. Acoust. Soc. Am., 91, 1648-1661]).